While glass plates which are used for windowpanes and the like are excellent in durability and optical properties, their sound damping and acoustic barrier performance (tan δ against flexural vibration), and shatter resistance, can be insufficient for many end uses. This is especially true if the application requires a light weight glazing element where thin glass plates are required.
Laminated glass is generally made by laminating two pieces of glass onto a plastic interlayer. The interlayer can greatly improve shatter resistance, sound damping and sound barrier performance of the laminate as compared to a comparably thick glass sheet, but often to the disadvantage of optical, flexural strength, stiffness and other properties.
In addition to high transparency and shatter resistance, many uses of laminated glass require a desirable combination of excellent sound insulating properties (both against a sound inside and outside) and flexural strength and stiffness under external load influences (for example, as glass building materials and automotive end uses).
Polyvinyl acetals represented by polyvinyl butyral are excellent in adhesion to or compatibility with various organic or inorganic base materials and solubility in organic solvents, and they are widely utilized as various adhesives or binders for ceramic, various inks, paints, and the like as well as safety glass interlayer films.
Films containing a polyvinyl acetal and a plasticizer are widely utilized as an interlayer film for laminated glass because they are excellent in adhesion to a glass. Laminated glass containing such interlayers can be made with good transparency, mechanical strength and flexibility, and shatter resistance.
In the case of constructing a glass in a place for which sound insulation is required, such as a window, the thickness of glass can be increased to enhance a sound insulating effect by weight, and a sound dampening interlayer can be laminated between two or more sheets of glass to enhance a sound insulating effect. In the latter method of using an interlayer film, the sound insulating properties of a windowpane are improved by using the interlayer film having a damping performance, and the interlayer film may also have ability of converting vibration energy into heat energy, thereby absorbing the vibration energy.
As one method of improving sound insulating properties, there is proposed a multilayer interlayer in which a polystyrene copolymer intermediate film between two exterior layers of a thermoplastic polymer material is laminated between two sheets of glass (see, for example, JP2007-91491A). As an example of a thermoplastic polymer material is mentioned a plasticized polyvinyl acetal-based resin and an ethylene-(meth)acrylic ester copolymer resin. However, the flexural strength of the laminate is not sufficient, making it unsuitable for uses under external load influences.
Other plastic materials have found wide use in safety laminates. For example, U.S. Pat. No. 7,951,865B1 discloses the use of an at least partially neutralized ethylene acid copolymer (ionomer) as an interlayer for preparing laminated safety glass. While these types of ionomers have excellent flexural strength and adhesion to glass properties, and good optical properties, laminates made from such ionomer interlayers do not possess good sound dampening properties.
As another method, there is proposed an interlayer film for laminated glass composed of polyvinyl butyral and having certain impact resistance and sound insulating properties (see, for example, U.S. Pat. No. 7,452,608B2). The interlayer film cam be single- or multilayer but, again, the flexural strength of the laminate is not sufficient for uses under external load influences.
In addition to the above, there has been much development in preparing multilayer films of polyvinyl acetal resins in which at least one of the intermediate polyvinyl acetal layers has sound dampening qualities. For example, there is proposed a method in which a sound insulating layer is constituted of three layers of PVB film (see, for example, US2013/0183507A1) and the like. However, the sound insulating properties of the proposed system was not maintained at a high level over a broad temperature range.
Other materials have been proposed to replace the intermediate polyvinyl acetal sound dampening layer, for example, a viscoelastic acrylic layer such as disclosed in U.S. Pat. No. 7,121,380B2, a layer containing a copolymer of styrene and a rubber-based resin monomer between layers containing a thermally adhesive resin such as disclosed in JP 2009-256128A, a layer containing a polyolefin such as disclosed in US2012/0204940A1, a layer containing an ethylene/vinyl acetate polymer such as disclosed in WO2015/013242A1, and a layer containing an ethylene acid copolymer such as disclosed in WO2015/085165A1.
In the transportation field, for example for the purpose of reducing the weight of a vehicle to improve fuel efficiency, thinning of glass is being advanced in recent years. However, when the glass is thinned, a coincidence threshold frequency (the coincidence threshold frequency means a lowest frequency in a frequency region where a coincidence effect in which the sound insulating performance falls in a high-frequency region as compared with that expected by the mass law) shifts toward the high-frequency side, so that the sound insulating performance in a high-frequency region is lowered. In the conventional laminated glasses using an intermediate film having sound insulating properties, the above-described phenomenon is liable to take place, so that improvements were required.
In addition, thinning the glass reduces the load-bearing capacity and stiffness of the laminate thus leading to structural reliability challenges in the design of light weight glazing.
In order to obtain laminated glass with improved sound insulating properties in a high-frequency region, there were proposed a method of adjusting a thickness of each layer of a multilayer interlayer and a hydroxyl group quantity of a polyvinyl acetal resin (see, for example, US2013/0183507A1), a method of selecting a polyvinyl acetal resin and a plasticizer in each layer of a multilayer interlayer such that a cloud point of a plasticizer solution obtained from a polyvinyl acetal resin and a plasticizer constituting each layer is a prescribed relation (see, for example, U.S. Pat. No. 8,741,439B2), a method of crosslinking a sound insulating layer (see, for example, JP2012-214305A), a method of using a plasticizer such that a difference in an SP value from a polyvinyl acetal resin is a prescribed value or more (see, for example, U.S. Pat. No. 8,883,317B2), and the like. However, the improvement of the sound insulating properties in a high-frequency region was not sufficient.
As indicated previously, polyvinyl acetals (and in particular polyvinyl butyral) are the predominant thermoplastic materials used as interlayers for glass laminates. These materials are used in a highly plasticized form, in other words, high amounts of plasticizers are added to polyvinyl acetals for use as interlayers.
In preparing a laminated glass, a heat treatment using an autoclave or the like is conducted. However, when a multilayer interlayer film with plasticized components is heated, an interlayer migration of plasticizer occurs, so that distribution of the plasticizer changes. After preparation of the laminated glass, the distribution of the plasticizer moves back towards the original state with a lapse of time at ordinary temperature; however, in view of the fact that the plasticizer migrates into other layer, physical properties, such as sound insulating properties, become unstable until equilibrium is reached. For that reason, in the case of preparing a laminated glass using the conventional multi-layered intermediate film, it is needed to retain the prepared laminated glass for a certain period of time until the sound insulating performance is stabilized. This retention time is problematic from the viewpoint of productivity, and an improvement is required in laminated glasses having a sound insulating function.
In addition, plasticizer equilibrium is dependent on temperature, so even in intermediate films for laminated glass having high sound insulating properties at ordinary temperature, there was involved such a problem that the sound insulating properties are largely lowered in the summer season or winter season. For that reason, a laminated glass capable of revealing the sound insulating properties at a high level over a broad temperature range was required.